Monster
Comet Promises Big Show

Using a very simple viewgraph simulator, teachers can explain
why comets take different shapes.

Materials:

Two transparencies

Colored pens
and drawing materials

A thick needle

A snap or round-headed
fastener

Construction:

Draw a 10-centimeter
(4-inch) diameter circle on one of the transparencies. This will simulate
the Earth's orbit around the Sun. Somewhere on the orbit, draw a colored circle
about 1 centimeter (half-inch) in diameter; this will represent Earth. Draw
the Sun at the center of the orbit. Cut the transparency about a centimeter
outside the Earth's orbit. You should now have a circular piece of transparency,
which you will later spin to represent the Earth's yearly motion.

On the second
transparency, draw a section of the comet's orbit. This should look like an
arc of an elongated ellipse (see photo). Mark the Sun's position. Draw several
comets along the path with their tails pointing away from the Sun. The tails
should be smaller when the comet is farther out. In preparing the viewgraph,
you may want to draw first on ordinary paper and later trace it onto the transparency.

Heat the point
of the needle about 5 seconds on a stove. Poke a hole through both transparencies
at the center of the Sun.

Place the Earth's
orbit transparency on top of the comet one. Fasten them with the snap. You
should be able to spin the Earth and change its position relative to the comet.

You can create
other viewgraphs that have a smaller scale and show several comet orbits or
constellations. Pupils can construct their own comet simulator. They can use
cardboard instead of a transparency for the comet's path.

Use:

Show how the
Earth's orbit is almost circular compared to the comet's. Explain that the
Sun is at the focus of the orbits of the planets, asteroids, and comets. Comet
orbits are highly elongated; this takes them far from the Sun. In fact, the
elongated orbit is why astronomers believe comets originally came from the
outer reaches of the solar system.

Explain that
the comet's tail always points away from the Sun and grows as it approaches
it. The tail on the viewgraphs is to scale: Comet tails really do stretch
tens or even hundreds of millions of kilometers. Actually, comets have two
tails. One, made of hot gas, point directly away from the Sun -- pushed back
by the force of the particles that stream outward from the Sun. The other
tail, made of dust left behind in the cometary orbit, is short and curved.

Turn Earth to
different positions and investigate how comets look from several vantage points.
We can see a comet head-on (its tail will look short) or on its side (the
tail will look long).

Explain that
sometimes we cannot see the comet, because it is behind the Sun. In January
and February, Comet Hale-Bopp will be very close to the Sun from our vantage
point.

Explain that
the tail changes size and that Earth can go through the tail.

Be sure to explain
that the orbit on the viewgraph is a projection. All bodies orbit in a plane,
but a comet does not necessarily orbit in same plane as Earth does.

JULIETA FIERRO is
in charge of astronomy popularization efforts at the Instituto de Astronom&iacutea
of the Universidad Nacional Aut&oacutenoma de M&eacutexico in Mexico City. One
of her many projects has been to work with homeless children in Mexico City --
the subject of an upcoming article in Mercury magazine. Her email address is julieta@astroscu.unam.mx.